1. Field of the Invention
The present invention generally relates to quadrature amplitude modulation (QAM) and particularly to nonlinear amplified quadrature amplitude modulation. More precisely, the invention relates to 16-QAM phase and amplitude modulation systems.
A diagram representing the possible relative phases and amplitudes of a 16-QAM system is represented in FIG. 1. It results from the cophasal composition of two four-phase modulated signals, the amplitude of one four-phase modulated signal being 6 dB below the other.
Systems for modulating the phase of a carrier wave by the digital pulses of a binary message have already been proposed. The better known of these are two-, four- or eight-phase systems. Thus, for a two-phase-state modulation, each bit having value 1 is represented by a carrier wave having a reference phase, i.e., phase 0, whereas each bit having value 0 is represented by a carrier wave having a phase shifted by .pi. compared to the phase of a binary bit. In the four-phase-state modulation case, a carrier wave takes each of four different phase values depending on the value assumed by each group of two successive bits whereupon the phase is one of four equi-spaced phases separated by .pi./2 of such wave.
For spectral reduction reasons, new generations of digital radio links make use of amplitude and phase multistate modulation; more particularly they use 16 amplitude-and-phase-state modulation. Multistate systems utilize a narrower passband and improve the efficiency in bits per second per Hertz. The 16-QAM constellation signal is actually impossible to amplify nonlinearly because it comprises an amplitude modulation component and a phase modulation component. Therefore, the transmitter of such a signal must operate at a level of 8 to 10 dB below saturation. This implies the need to use microwave components having extensive power margins and a very low energy yield whence high consumption. A linearly amplified 16-QAM modulator is therefore not very reliable and inefficient energy-wise.
An improvement was put forward in the IEEE transactions and communications document Vol. Com. 30 No. 3 of March 1982 by DOUGLAS H. MORAIS, and KAMILO FEHER. In this article, the 16 phase and amplitude state transmitting system is regarded as the combination of two independently operated four-phase modulators. The output signals of the modulators are amplified separately by a nonlinear amplifier. This form of amplification, which has a tendency to convert amplitude-modulation to phase-modulation, is made possible solely because the 16-QAM signal is built up by summation after amplification of both four-phase-state modulated carriers.